Astronomy%20100%20Tuesday,%20Thursday%202:30%20-%203:[e-mail]%20www.xanga.com/astronomy100

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Astronomy 100Tuesday, Thursday 230 - 345
pmTom Burbinetburbine_at_mtholyoke.eduwww.xanga.
com/astronomy100
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Help Desk

• There is an Astronomy Help Desk in HAS 205.  It
  will be open from Monday through Thursday from
  7-9 pm.

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E mc2 and KE 1/2 mv2 HW

• Purpose of this Homework
• To teach you that if you can get lots of energy
  from a small amount of material if you can turn
  all the matter into energy
• You need a much bigger mass to produce the same
  amount of energy through kinetic energy (through
  impact with a velocity)

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• The nuclear bomb dropped on Hiroshima released
  8.4 x 1013 joules of energy. What would be the
  mass of nuclear material that would produce this
  amount of energy if you assume all the nuclear
  material was converted to energy? Show your
  work.
• E mc2
• ME/c2
• c 3 x 108 m/s
• c2 9 x 1016 m2/s2
• M 8.4 x 1013 J / 9 x 1016 m2/s2
• M 9.3 x 10-4 kg

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• An asteroid is on a collision course with Earth?
  What must the mass of the asteroid be to produce
  that much energy (8.4 x 1013 joules) if it hits
  the Earth with a velocity of 20 km/s (20,000
  m/s)? Show your work.
• KE kinetic energy
• KE ½ m v2
• m KE/(½ v2)
• m 2KE/v2
• m 2 8.4 x 1013 J/(2000020000)
• m 4.2 x 105 kg

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Also

• You need to list units for answer!!!!!!
• Like kilograms for the previous questions

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Element Homework

• So you learn that there are lots of elements with
  different properties
• And each element has a number of isotopes
• Isotope same number of protons, different
  number of neutrons

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Latest Homework

• Was not done to torture you
• Was done so you realize that different planets
  have different accelerations due to gravity and
  different escape velocities
• How would that information be used on a test?

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For example,

• A body is the same size as the Earth but has
  twice the Earths mass. What would be the
  acceleration of gravity on this body?
• A) 4.9 m/s2
• B) 9.8 m/s2
• C) 19.6 m/s2
• D) 2.45 m/s2
• E) 39.2 m/s2

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For example,

• A body is the same size as the Earth but has
  twice the Earths mass. What would be the
  acceleration of gravity on this body?
• A) 4.9 m/s2
• B) 9.8 m/s2
• C) 19.6 m/s2
• D) 2.45 m/s2
• E) 39.2 m/s2
• Because of formula F G M1 M2
• 
  d2

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Homework Assignment(Due Thursday March 3rd)

• Make up a test question
• Multiple Choice
• A-E possible answers
• 1 point for handing it in
• 1 point for me using it on test
• The question needs to be on material that will be
  on the March 10th exam

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Next Homework (due Tuesday, March 1st)

• In Joules, calculate the typical energy of one
• Gamma ray
• X-ray
• Ultraviolet light
• Visible light
• Infrared light
• Radio wave
• photon

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Light

• Light is a form of energy

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Light

• These are all forms of light
• Gamma rays
• X-rays
• Ultraviolet light
• Visible light
• Infrared light
• Radio waves

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Light

• Can act as a particle
• Can also act as a wave

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Particle aspect

• Particles called photons stream from the Sun and
  can be blocked by your body

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Wave aspect
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Thomas Young Experiment

• http//micro.magnet.fsu.edu/primer/java/interferen
  ce/doubleslit/

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Characteristics of waves

• velocity wavelength x frequency
• Wavelength distance
• Frequency cycles per second hertz

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For light

• c wavelength x frequency
• In vacuum, speed of light stays the same
• So if wavelength goes up
• Frequency does down
• f frequency
• ? wavelength
• c ? x f

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Show animation

• Electromagnetic spectrum

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Calculations

• c ? x f
• So if the wavelength is 1 x 10-12 m
• 3 x 108 m/s 1 x 10-12 m f
• f 3 x 108 m/s/1 x 10-12 m
• f 3 x 1020 s-1 3 x 1020 Hz

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Calculations

• c ? x f
• So if the frequency is 1 x 1015 Hz
• 3 x 108 m/s ? 1 x 1015 Hz
• ? 3 x 108 m/s/1 x 1015 Hz
• ? 3 x 10-7 m

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Energy of light

• Energy is directly proportional to the frequency
• E h f
• h Plancks constant 6.626 x 10-34 J/s
• since f c/?
• Energy is inversely proportional to the
  wavelength
• E hc/?

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Higher the frequency, Higher the energy of the
photon Higher the wavelength, Lower the energy of
the photon
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Calculations

• What is the energy of a radio wave with a
  frequency of 1 x 107 Hz?
• E h f
• h Plancks constant 6.626 x 10-34 J/s
• E 6.626 x 10-34 J/s 1 x 107
• E 6.626 x 10-27 J

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Calculations

• What is the energy of a gamma ray photon with
  wavelength of 1 x 10-15 m
• E hc/?
• h Plancks constant 6.626 x 10-34 J/s
• E 6.626 x 10-34 J/s 3 x 108 m/s / 1 x 10-15 m
• E 1.99 x 10-10 J

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Next Homework (due Tuesday, March 1st)

• In Joules, calculate the typical energy of one
• Gamma ray
• X-ray
• Ultraviolet light
• Visible light
• Infrared light
• Radio wave
• photon

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So why are some types of radiation dangerous?

• Higher the energy, the farther the photons can
  penetrate
• So gamma and X-rays can pass much more easily
  into your the body
• These high-energy photons can ionize atoms in
  cells
• Ionization means removes electrons from an atom

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More dangerous
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When you measure an astronomical body

• You measure intensity
• Intensity amount of radiation

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How do you use light to determine what is in an
astronomical body?
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electrons
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Energy levels where an electron can reside To go
to a higher energy level, an electron needs to
gain energy To go to a lower energy level, an
electron needs to lose energy
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Rules

• An electron can not jump to a higher energy level
  unless it gains energy from somewhere else
• Absorbs a photon
• Gains kinetic energy from an impacting particle
• To go to a lower energy level, the electron must
  lose energy
• Emits a photon
• Electron jumps can occur only with the particular
  amounts of energy representing differences
  between possible energy levels

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So which of these transitions is not possible

• A
• B
• C
• D
• E

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So which of these transitions is not possible

• A
• B
• C
• D
• E

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Show animation

• Production of emission lines

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Heated hydrogen gas Emission line spectrum
White light through cool hydrogen gas Absorption
line spectrum
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Show animation

• Production of absorption lines

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Types of spectra

• Emission radiation is emitted at characteristic
  wavelengths
• Material is hot so electrons keep on bumping
  into each other and transferring kinetic energy
  toe ach other so they jump between particular
  energy levels
• Absorption radiation is absorbed at
  characteristic wavelengths
• Radiation passes through the material

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So why is this important

• Different elements have different number of
  electrons
• Different elements have different energy levels
  for their electrons

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So

• Different elements can absorb light at specific
  energies
• Different elements can emit light at specific
  energies
• So if you can measure the wavelength of the light
  from an astronomical body, you can determine
  whats in it

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Emission line spectra
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Show animation

• Composition of a mysterious gas

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How can you all this to determine velocities?

• Doppler Shift The wavelength of light changes
  as the source moves towards or away from you
• Since you know the wavelength position of
  emission or absorption features
• If the positions of the features move in
  wavelength position, you know the source is moving

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So

• Source moving towards you, wavelength decreases
• Source moving away from you, wavelength increases

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Questions?